Optimization Of Thermoelectric Properties Of N-type Mg₂Si_1-xSn_x Based Materials Through Doping As Well As The Adjustment And Control Of Band Structure

Medium-temperature (500-900K) thermoelectric materials could be used for reusing the exhaust heat of automobile and industrial waste heat and converting them into available electricity, and were prospective to increase the conversion efficiency of the fossil energy. Thus the investigation and development of these materials were strategically important for saving energy and reducing carbon emission in our country. Being one type of important medium-temperature thermoelectric materials, Mg2Si1-xSnx-based solid solutions have attracted considerable interest as prospective thermoelectric materials for waste heat recovery because of their abundant and low cost chemical constituents, not containing scarce Te element, environmentally friendly, low density and so on. The thermoelectric properties of n-type Mg2Si1-xSnx based solid solutions were still relatively low up till now, and their highest ZT value was about1.1. The main reasons that limited the enhancement of ZT values of these materials were listed below:Mg content hard to control precisely and its influence on thermoelectric properties remaining unknown, the carrier concentration and band structure lack of enough optimization. In addition, the investigation of long-term thermal stability of these materials at high temperatures was one of the key points for the application of thermoelectric power generation, however there also lacked related systematic study of thermal stability of these materials at the moment.Based on the above difficulties, in this research n-type Mg2Si1-xSnx based solid solutions were adopted for the following issues:a) realizing the precise control of Mg content and exploring the influences on point defects, electron concentration and thermoelectric properties of n-type Mg2Si1-xSnx caused by Mg content deviating from its stoichiometric amount; b) adjusting the electron concentration through Sb or Bi doping and the variation of Mg content, and investigating the effect of electron concentration on thermoelectric properties of n-type Mg2Si1-xSnx; revealing the correlation between conduction band structure and electrical properties by means of band structure calculation along with systematical experimental study, and optimizing the electrical properties of n-type Mg2Si1-xSnx through modifying and optimizing their band structure; a primary exploration of thermal stability for n-type Mg2Si1-xSnx based solid solutions. The major research results were listed below:The effective management of the compositions for Mg2Si1-xSnx based solid solutions, especially for the Mg amount, was realized by optimizing the synthesis process and by fixing the parameters of the process. The influence of Mg content combined with Sb doping amount on the electron concentration, point defects and thermoelectric properties of n-type Mg2Si1-xSnx based materials were investigated in this research and showed that, the excess of Mg could introduce intrinsic points defects like interstitial Mg and vacancies Si in the crystal lattice which behaved as the electrons donor and significantly enhanced the electron concentration and power factor of these materials. On the basis of Sb doping, the electron concentration of n-type Mg2Si1-xSnx based solid solutions was increased by2-10times, and by1.3～1.8×1020cm-3with Mg amount increased by7-12%. However, the electron concentration varied slightly with Mg content in the case of non-Sb doping. Thermal properties measurement indicated that, the deviation of Mg content from stoichiometric amount did not affect the lattice thermal conductivity of n-type Mg2Si1-xSnx based materials.The influence of Sb/Bi doping on the thermoelectric properties of n-type Mg2Si1-xSnx based solid solutions were discussed in this research. Sb/Bi doping, with the amount in the range of0-4%, could cause an enhancement by one to two orders of magnitude in the electron concentration, the electrical properties as well as the power factor. The regulating action of Bi doping for electrical properties was similar to Sb. Under the condition of heavily-doping, the electron concentration of n-type Mg2Si1-xSnx based materials has nearly reached a saturated point with further increase of Sb/Bi amount. Thus, in order to effectively optimize the electron concentration and power factor of n-type Mg2Si1-xSnx based solid solutions, we not only needed Sb/Bi doping but also needed excess addition of Mg. In addition, Sb doping could not reduce the lattice thermal conductivity of n-type Mg2Si1-xSnx based materials while Bi had larger suppression effect on the lattice thermal conductivity compared to Sb, which was caused by much larger atomic mass of Bi. Due to the large enhancement of electrical properties and somewhat decrease in the lattice thermal conductivity through Bi doping, Mg2Si0.4Sn0.6based solid solution doped with3%Bi possessed the largest ZT value of1.40at high temperatures.Band structure calculations based on first-principles-theory and systematic experimental studies indicated that, the edge of light conduction band and heavy conduction band converged in energy with increasing Sn/Si ratio, and degenerated at x=0.65-0.68. The degeneration of conduction band structure would result in an enhancement on density-of-states effective mass and Seebeck coefficient, but had no detrimental impacts on the carrier mobility. Low-temperature electronic heat capacity data and thermoelectric properties measurement results confirmed the converging and degenerating features of the conduction bands. The density-of-states effective mass, Seebeck coefficient and power factor of n-type Mg2Si1-xSnx based solid solutions were enhanced with the increase of Sn/Si ratio, and gained the largest value at Mg2Sio.3Sno.7where two conduction bands nearly degenerated. Due to the lowest lattice thermal conductivity was obtained at Mg2Si0.4Sn0.6, n-type Mg2Si1-xSnx based solid solutions gained the largest ZT values at x=0.6-0.7. Experimental results also revealed that, n-type Mg2Si1-xSnx based solid solutions with different Sn/Si ratio obtained the optimum ZT values at1.6×1020<n<2.5×1020cm-3. The research suggested that striving to achieve band degeneracy by means of compositional variations was an effective strategy for enhancing properties of thermoelectric materials.The investigation of thermal stability of n-type Mg2Si1-xSnx with x=0.5,0.6,0.65and0.70at573～773K for1～3weeks showed that, the thermal stability of these materials was closely connected with the miscibility gap of pseudo-binary phase diagram of Mg2Si-Mg2Sn. The ZT values for these n-type Mg2Si1-xSnx materials remained nearly unchanged or enhanced in some extent in the annealing process. The thermoelectric properties of Mg2Si0.5Sn0.5based solid solutions, which were positioned in the miscibility gap of the phase diagram, did not change after annealing while the phase structure changed in this process. For Mg2Si0.4Sn0.6based solid solutions, the electrical and thermal properties varied a lot before and after annealing while phase structure remained unchanged, resulting from this component situated in the boundary area of the miscibility gap. Due to Mg2Si1-xSnx with x=0.65and0.7nearly or totally positioned in the continuous solid solutions region of the phase diagram, these materials were stable in both the phase structure and thermoelectric properties in the annealing process. Therefore, Mg2Si1-xSnx with x=0.65and0.7were confirmed to possess both the excellent thermoelectric properties and thermal stability, and were most likely to be the best choice in n-type Mg2Si1-xSnx for thermoelectric power generation in the range of500～800K.